Culvert Hydraulics Calculator

Inlet vs outlet control — read both

A culvert can be controlled by either its entrance geometry (inlet control) or by the friction + velocity head along the entire barrel (outlet control). The actual headwater equals whichever computed value is higher. Steep, smooth culverts are usually inlet-controlled; long, rough, or flat culverts with high tailwater are usually outlet-controlled. The only way to know is to compute both.

Inlet coefficient ranges (FHWA HDS-5 Table 4-1)

This calculator uses the submerged form (Form 2). It's accurate when Q/(A·D^0.5) is greater than about 4, which is the typical design-flood condition. For low-flow analyses, the unsubmerged form gives slightly different (typically lower) HW.

• Concrete pipe, square edge w/ headwall: c = 0.0398, Y = 0.67, K_e = 0.5
• Concrete pipe, groove end w/ headwall: c = 0.0292, Y = 0.74, K_e = 0.2
• Concrete pipe, groove end projecting: c = 0.0317, Y = 0.69, K_e = 0.2
• Corrugated metal, headwall: c = 0.0379, Y = 0.69, K_e = 0.5
• Corrugated metal, mitered to slope: c = 0.0463, Y = 0.75, K_e = 0.7
• Corrugated metal, projecting: c = 0.0553, Y = 0.54, K_e = 0.9

The entrance loss coefficient K_e is the kinetic-energy multiplier on V²/(2g) for entrance contraction losses. Square edges are worse than grooves; projecting inlets are worst. Groove-end inlets with a headwall are the best cheap inlet.

When to use each formula

For preliminary culvert sizing on a typical road crossing, this calculator gets you within ±10% of a full FHWA HY-8 analysis at the design flow. For more rigor, you need:

• FHWA HY-8 (free download from FHWA website) — full inlet/outlet control with unsubmerged + submerged transitions, performance curves, fish-passage analyses.
• HEC-RAS culvert routine — for road overtopping, multi-barrel arrangements, complex tailwater conditions.
• Agency-specific design tools — your state DOT may require a specific calculation method for highway crossings.

The simplifications in this calculator: full-flowing barrel for outlet control (assumes Q is enough to fill the pipe — typical at design flood), submerged-form inlet control only, no critical-depth or partial-flow analysis. Document your assumptions when using this for a real submittal.

Common gotchas

• Tailwater can dominate. If TW is above the outlet crown, the culvert is operating with a submerged outlet. The energy equation here handles that, but make sure your TW estimate from the receiving channel's normal depth is correct.
• Mitered inlets aren't free. Mitering a CMP to match a fill slope looks clean but increases the inlet loss coefficient by 40%+ over a headwall. The hydraulic cost is real.
• Multi-barrel culverts split flow. If you have two 36" pipes side-by-side, each carries Q/2 — but the inlet and outlet head losses scale roughly with V², so the per-barrel HW is significantly less than for a single 36" handling Q. Run each barrel separately.

Reference: FHWA Publication FHWA-HIF-12-026 (2012). Hydraulic Design of Highway Culverts (HDS-5), 3rd ed. Polynomial coefficients from Table 4-1.